Remember water weighs 8.34 pounds per gallon. 12 gallons is just over a hundred pounds. I suspect the unit itself with battery probably weighs at least 35 pounds empty Don't plan on going uphill--nor lifting it onto your pickup
Perhaps your shoulders would feel a bit more rested if you filled your backpack only half way--and used a more concentrated solution so you would cover the same sqft per fill. Also consider an electric backpack.
Matter of fact--has anybody been able to devise a system to carry your electric backpack in your spreader--easy push and spot spray.